The invention relates generally to communication apparatus and, more particularly, to such apparatus including systems for the active suppression of undesired signals.
Modern communication and signal collection systems include wideband analog-to-digital (A to D) converters for changing a converted version of a transmitted analog signal into a digital signal to facilitate digital processing thereof. Such A to D converters can only handle a predetermined amount of total signal input power before they go into saturation and begin clipping the time domain signal thereby resulting in undesirable spectral content in the output signal of the converter. Thus strong undesirable Signals Not Of Interest (SNOI""s) tend to undesirably capture the dynamic range of such A to D converters. This effect suppresses the detection capability for the (Signals of Interest) SOI""s that are weaker in power. Such SOI""s may be of unknown frequency offset with respect to the SNOI""s. Thus this problem can be particularly acute in wideband systems receiving SNOI""s of large magnitudes. Such SNOI""s can have known characteristics such as frequency or modulation type, for example.
There are several analog and digital prior art systems which address the foregoing problem by suppressing the magnitude of such SNOI""s to result in an increase in the relative magnitude of the SOI""s applied to the system A to D converter. Some of these prior art analog systems employ complex phased array antenna systems or tunable notch filters. An exemplary prior art solution using multiple antennas is disclosed in U.S. Pat. No. 4,989,262 issued to Saito on Jan. 29, 1991 and another prior art solution showing a plurality of filters is disclosed in U.S. Pat. No. 5,524,125 issued to Tsujimoto on Jun. 4, 1996, the entire contents of each of which are hereby incorporated herein by reference.
Since the SNOI""s can be at any frequency in the selected band or have any modulation characteristic, some prior art targeted coherency systems require complex demodulators and remodulators unique to each form of modulation to be cancelled. Still other prior art digital approaches rely on simple Fast Fourier Transform (FFT) bin or channel excising which doesn""t provide the degree of selective discrimination required for some applications. All of the above problems with the prior art solutions makes them expensive and difficult to implement and otherwise unsuitable for many applications.
Therefore, there is a need for an interference or SNOI suppression system which is capable of operating with a single simple antenna and which doesn""t require targeted coherency and tracking techniques or notch filters. There is also a need for a SNOI suppression system that is essentially modulation independent and that is capable of highly selective discrimination. A further need is to provide a SNOI suppression system that also provides automatic gain control (AGC).